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arXiv:quant-ph/0512128 · Physical Review A

Long-distance quantum communication with neutral atoms

Mohsen Razavi, Jeffrey H. Shapiro

Published Apr 5, 2006• 51 citations
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Abstract

The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for long-distance quantum communication with atomic ensembles is analyzed. Its fidelity and throughput in entanglement distribution, entanglement swapping, and quantum teleportation is derived within a framework that accounts for multiple excitations in the ensembles as well as loss and asymmetries in the channel. The DLCZ performance metrics that are obtained are compared to the corresponding results for the trapped-atom quantum communication architecture that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT and NU). Both systems are found to be capable of high-fidelity entanglement distribution. However, the DLCZ scheme only provides conditional teleportation and repeater operation, whereas the MIT-NU architecture affords full Bell-state measurements on its trapped atoms. Moreover, it is shown that achieving unity conditional fidelity in DLCZ teleportation and repeater operation requires ideal photon-number resolving detectors. The maximum conditional fidelities for DLCZ teleportation and repeater operation that can be realized with nonresolving detectors are 1/2 and 2/3, respectively.

DOI
10.1103/physreva.73.042303

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The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for long-distance quantum communication with atomic ensembles is analyzed. Its fidelity and throughput in entanglement distribution, entanglement swapping, and quantum teleportation is derived within a framework that accounts for multiple excitations in the ensembles as well as loss and asymmetries in the channel. The DLCZ performance metrics that are obtained are compared to the corresponding results for the trapped-atom quantum communication architecture that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT and NU). Both systems are found to be capable of high-fidelity entanglement distribution. However, the DLCZ scheme only provides conditional teleportation and repeater operation, whereas the MIT-NU architecture affords full Bell-state measurements on its trapped atoms. Moreover, it is shown that achieving unity conditional fidelity in DLCZ teleportation and repeater operation requires ideal photon-number resolving detectors. The maximum conditional fidelities for DLCZ teleportation and repeater operation that can be realized with nonresolving detectors are 1/2 and 2/3, respectively.
The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for entangling distant atomic ensembles is addressed and analyzed. Its performance, in terms of fidelity and throughput, is compared to that of the quantum communication architecture using trapped rubidium-atom quantum memories that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT/NU). It is shown that the DLCZ protocol for entanglement distribution achieves a better throughput versus distance behavior than does the MIT/NU architecture, with both being capable of high entanglement fidelities. The DLCZ scheme also admits to a conditional teleportation scheme based on its entangled atomic ensembles, whereas the MIT/NU architecture affords unconditional teleportation based on its trapped-atom quantum memories. It is shown that achieving unity fidelity in DLCZ teleportation requires photon-number resolving detectors; the maximum teleportation fidelity that can be realized with non-resolving detectors is 1/2.
Quantum communication deals with absolutely secure transfer of classical messages by means of quantum cryptography or faithful transfer of unknown quantum states by means of quantum teleportation between distant sites. The essential element for quantum communication is to create remote entangled pairs of high fidelity. The main difficulty to create such a distant entangled pair is that both photon loss and decoherence grow exponentially with distance. However, with the help of quantum repeater, it is demonstrated that both the entanglement sources and the time needed to create a single distant entangled pair of high fidelity could scale polynomially with the communication length. Here we propose a robust quantum repeater by extending the original Duan-Lukin-Cirac-Zoller (DLCZ) scheme based on atomic ensembles and linear optics. Our protocol entails the advantage of two photon interference, which is more robust than single photon interference used in the DLCZ scheme. In comparison with our previous proposal, the entangled memory qubits are manipulated across long distance, and local entanglement pairs are avoided. Our proposal provides an exciting possibility for realistic long distance quantum communication.
We present a detailed analysis of a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L.M. Duan et al. Nature (London) 414, 413 (2001)]. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance interferometric stability requirements by about seven orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons, thereby removing the weakest point in the DLCZ scheme. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.
A feasible route toward implementing long-distance quantum key distribution (QKD) systems relies on probabilistic schemes for entanglement distribution and swapping as proposed in the work of Duan, Lukin, Cirac, and Zoller (DLCZ) [Nature (London) 414, 413 (2001)]. Here, we calculate the conditional throughput and fidelity of entanglement for DLCZ quantum repeaters by accounting for the DLCZ self-purification property in the presence of multiple excitations in the ensemble memories as well as loss and other sources of inefficiency in the channel and measurement modules. We then use our results to find the generation rate of secure key bits for QKD systems that rely on DLCZ quantum repeaters. We compare the key generation rate per logical memory employed in the two cases with and without a repeater node. We find the crossover distance beyond which the repeater system outperforms the nonrepeater one. That provides us with the optimum internode distancing in quantum repeater systems. We also find the optimal excitation probability at which the QKD rate peaks. Such an optimum probability, in most regimes of interest, is insensitive to the total distance.
In this Letter we propose a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)10.1038/35106500]. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance stability requirements by about 7 orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.